A phased array antenna is a network of radiating elements, each of which is usually non-directive (i.e. isotropic), having a cooperative radiation pattern that is a highly directive beam. Whereas conventional radar antennae have to be mechanically steered to meet beam directing requirements, a phased array achieves the same effect electronically by changing the phase of the signal radiated by each element. Thus, accurate beams are formed and directed simply by driving each element of the array with a signal having an appropriate phase. As a further advantage, electronic steering is much faster than mechanical steering.
The flexibility of electronic steering provided by phased arrays requires individual control of each element. In an array having N elements, each of the elements is driven with a different phase of the same signal. In conventional systems, a single microwave signal is split into N equal signals, and a phase shifting network is provided for each radiating element for individual phase control. For large arrays, however, the cost and complexity of the power splitting network become limiting factors, and the computation required to calculate the array phase distribution for a desired radiation pattern becomes a non-trivial problem.
An optical beamforming network (OBFN) has been described by G.A. Koepf in "Optical processor for phased array antenna beam formation," SPIE, Vol. 477 (1984), as a system that addresses the limitations of large arrays. In an OBFN, microwave hardware is replaced by more wieldy fiber optics and modulation/demodulation devices. In addition, the problem of computing the phase of each array element is addressed with a simple coherent oPtical system using Fourier optics in conjunction with spatial light modulators. However, a high quality Fourier transforming lens must be used because aberrations produced by the lens cause inaccuracies in beam pointing and broadening. Thus, there is a need for improvements in the basic OBFN to ease alignment problems and to nullify the requirement for high quality optics.